Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
In order to reduce fuel consumption, some vehicles include hybrid powertrains which utilize energy storage to supplement the power produced by the internal combustion engine. These powertrains permit the vehicle to operate a portion of the time with the engine off and the remainder of the time at torque levels at which the engine is more efficient. Hybrid powertrains also enable the capture and later use of energy that would otherwise be dissipated by the braking system.
One form of hybrid powertrain utilizes an Integrated Starter Generator (ISG). Unlike a conventional starter motor, an ISG is designed to remain fixed to the crankshaft while the engine is running. As such, it can provide a number of functions such as recapture of braking energy and provision of supplemental torque that are not feasible for a conventional starter motor. Also, an ISG is typically capable of starting the engine quicker than a conventional starter motor, enabling engine shut down when the vehicle stops during a drive cycle and automatic restarting in response to release of the brake pedal. ISGs are typically mounted between the engine and the transmission, increasing the length of the powertrain and potentially decreasing powertrain stiffness.